The advent of digital circuit implementation to the electrical distribution and control field has resulted in combining several electronic functions within a single modular enclosure. One example of a circuit interrupter having supplemental protective relay function is found in U.S. Pat. No. 4,672,501 entitled "Circuit Breaker and Protective Relay Unit".
To provide a continuous sample of the current level within an associated electrical distribution system, a current transformer is connected within the circuit interrupter, as described within U.S. Pat. Nos. 4,591,942 and 5,321,378 both entitled "Current Transformer Assembly". The current transformers as employed therein also derive operating power from the circuit current to power-up the electronic components within the circuit interrupter electronic trip unit. It has been found advantageous to use a single iron core current transformer to both sense the circuit current along with providing operational power to the electronic trip unit in higher ampere-rated circuit interrupters. To prevent the iron cores from becoming saturated at higher current levels, expensive magnetic steel laminates are used and the laminates are sized to allow short circuit current sensing without causing the cores to saturate.
U.S. Pat. No. 4,796,148 entitled "Current-Sensing Arrangement Utilizing Two Current-Sensing Signals" teaches the use of a separate air core transformer and a separate iron core transformer to increase the current sensing range when the iron core saturates.
U.S. Pat. No. 4,297,741 entitled "Rate Sensing Instantaneous Trip Mode Network" describes the use of an iron core transformer for sensing ordinary current overload levels along with a separate air core transformer to sense short circuit currents.
U.S. Pat. No. 3,846,675 entitled "Molded Case Circuit Breakers Utilizing Saturating Current Transformers" teaches the use of iron core transformers for providing operating power to the trip unit and separate air core transformers for monitoring the circuit current.
In lower ampere-rated electronic circuit interrupters, the current transformer size constraints require the use of expensive core steel laminations to optimize transformer action with the least possible amount of material without reaching saturation when such current transformers are used for both sensing circuit current as well as powering up the electronic trip unit circuit. It would be economically desirable to perform such sensing and power-up functions by use of a single modular transformer design for all the reasons given earlier. One such modular design is described within U.S. patent application Ser. No. 08/358,493 filed 19 Dec. 1994 entitled "Modular Current Transformer for Electronic Circuit Interrupters" wherein separate iron core and air core transformers are used to sense circuit current within a protected circuit while providing operating power to the circuit interrupter electronic trip unit. U.S. patent application Ser. No. 08/735,719 entitled "Self Powered Axial Current Sensor" filed on 23 Oct. 1996 describes a compact current transformer arrangement wherein the current sensor coil is arranged within the transformer core and the power generating coil is arranged outside the transformer core. U.S. Pat. No. 5,774,320 entitled "Modular Current Transformer for Electronic Circuit Interrupters" describes a concentric arrangement of an air core transformer within an iron core transformer for providing current sampling to an electronic trip unit along with operating power. Tile advent of such compact and inexpensive current transformers now allows the use of circuit interrupters within lower ampere commercial and industrial environments.
One purpose of the invention is to provide a compact circuit interrupter employing an electronic trip unit whereby the operating power to the trip unit is provided by means of an iron core and the current sensing is provided by means of an air core both within a compact common current transformer assembly.
A further purpose of the invention is to provide means for reducing the operating temperature of the current transformer to allow compact insertion within smaller frame circuit breakers without overheating.